In communication systems that utilize voice channels for data transmissions it is necessary to mute the data in the receiver to prevent the radio user from hearing it. In the prior art, data muting in receivers typically has been accomplished by detecting the presence of a data signal and then muting the output of the receiver so that it is not heard. Many different techniques exist for detecting the presence of the data signal, such as, for example, those described in U.S. Pat. Nos. 3,758,860, 3,939,431, 4,197,502, 4,233,565, 4,229,822, and 4,430,742. Most of the foregoing prior art techniques require the reception of several bits in order to detect the data signal. Since a finite amount of time is needed for the detection of data, a short data burst is always passed to a listener.
Some systems use a data operated squelch (DOS) tone as a preamble to the data to inform the receiver circuitry of an arriving data signal. This DOS tone is detected via a detector circuit which subsequently acts to mute the speaker so as to prevent the user from hearing the received data. FIG. 1 shows a block diagram 100 of one such mute circuit.
A demodulated signal 102 is routed to a buffer 106 and a detector 108 via a signal line 104. The output of the buffer 106 is coupled to the input of an audio mute gate 112. The detector 108 detects the presence of data or DOS tone and directs the mute gate 112 via the gate control line 110 to open so as to prevent the gated audio 114 from getting to an amplifier 115 and a speaker 116.
In general the detector 108 takes a finite amount of time to detect the presence of a DOS tone and assert the gate control line 110. In addition, there is a finite reaction time for the audio mute gate 112 to react to the state of the gate control line 110. During this finite time period, however short, a data burst will get to the speaker 116. FIG. 2 shows a timing diagram depicting the passage of some data through the audio mute gate 112. Time t.sub.a 202 is the reaction time of the detector 108 when data is present. Time t.sub.b is the attack time of the audio mute gate 112. Times t.sub.c and t.sub.d are the data absent detect time and the audio mute gate release time respectively. As is apparent from the timing diagram 200, a data burst 210 equal in time to the sum of t.sub.a 202 and t.sub.b 204 is passed through the audio mute gate 112 and heard on the speaker 116. Furthermore, a portion of the voice signal equal in time to the sum of the two times t.sub.c 206 and t.sub.d 208 is missed.
The data burst 210 and the missing of a portion of the voice signal are annoying to the user and are therefore undesirable. It is clear that a need exists for a means to differentiate the data signals from voice and prevent them from reaching the speaker 116. Several means are available to provide a solution to this problem, however all with various deficiencies.
Receivers of some communication systems assume that data is not present and then mute the speaker on the detection of data. This is depicted in FIG. 2. Other systems assume that data is present and mute the audio. Upon detection of no data (voice), the system would unmute the audio. The problem with this approach is that the first part of the transmission is muted while determining whether it is data or not. Another problem with this solution is that in some data/voice systems there is a post-Push To-Talk (post PTT) ID sent with each transmission. In such systems, portions of the post PTT ID will be heard on the speaker due to the finite time required for the detection of data.
A second solution would use a high Q notch filter in systems where DOS tones are employed. Since DOS tones are audio tones in the voice spectrum a portion of the voice spectrum is filtered as well. The impact of this filtering of the voice spectrum varies from person to person. While it would not render the voice message unintelligible, it would distort the sound of the person to the point where they could be unrecognizable.
Therefore, there exists a long felt need for a means to effectively, and without any side effects, eliminate the annoying data bursts heard on a speaker in a data/voice communication system.